1. Field of the Invention
This invention relates generally to voltage converters and more particularly to a circuit for correcting the power factor of a boost-type voltage converter.
2. Description of the Related Art
Simple capacitor rectification circuits have generally been used to obtain DC power from alternating current ("AC") power sources. However, in such rectification circuits, the input current flows exclusively in pulses at the peak portion of the input AC voltage, thereby degrading the power factor. Also, because a combination of resistive (R), inductive (L) and capacitive (C) components appear in various electric devices, the current and voltage from the power source are out of phase with respect to each other and the voltage is subjected to distortion.
If high-speed switching methods are employed to control electric appliances, it causes noise in the distribution system. Such noise results in interference between electric devices connected to the same power line. In order to decrease the effect exerted on the power supply voltage by the current flowing through the electrical appliance itself, the electrical appliance may be designed to maintain a high input power factor.
It is well known that a continuous current mode (CCM) control type power factor correction circuit is suitable for use in high power factor applications. Conventional CCM control type power factor circuits can be classified into three types: peak-current detection type, variable hysteresis type, and mean current control type. All of these circuits, however, have disadvantages when used for obtaining a high power factor.
Peak-current detection type power factor circuits tend to produce distortion which prevents accurate power compensation. Variable hysteresis type power factor circuits tend to present difficulties in controlling frequency during low voltage applications. The circuitry required for implementing a mean current control type system tends to be very complicated.
Also, in conventional power factor correction circuits, a voltage feedforward circuit is employed for sensing the input voltage so as to allow the power factor circuit to maintain the input current in phase with the input voltage. However, voltage feedforward circuits add complication to the power factor correction circuit.
Accordingly, a need remains for an improved technique for controlling the power factor in a switching power supply.